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Rex Burns, P.E. and Sob Sf P.E, Final Repon . Hydrologic A1.~ysis of tile Cooper Slough Within, Larimer COUw.y, Colorado Page 3 Creek basin for each stcnn event, but did not distinguish what portion of the discharges flow down the highway medillIl and what portion flows over the southbound lanes into the Cooper Slough basin. Therefore" the area of overtopping was suIiley'ed to determine the distribution of the overflow. The surveyed section is shown in Appendix A. A nonnal depth equation was used to determine the amount of discharge that flows down the highway median to retum to Boxelder Creek; and a weir equation was used to determine the amount of discharge that overtops the southbound lan~~ and flows into the Cooper Slough basin. A rating curve table of water surface elevation versus discharge can be found in Appendix A. The following table lists the updated overtopping flows at State Highway 14. TABLE 2 1-25 OVERTOPPING DISCHARGES AT STATE HWY 14 5TO&'\:1 RECURRENCE INTERVAL 10- year 50-year IOO-year 500-year TOTAL FLOW ( cfs) 275 530 726 1136 FLOW TO COOPER SLOUGH .-kLll._ 29 173 301 590 now TO MEDIA.N ( cftl 246 357 425 546 The values listed in the "fI<)W to Cooper Slough" column above, as well as the discharges listed in Table I (except for State ::1ighway 14) are used to compute the inflow hydro graphs that will be added to the updated Cooper Slough SWMNf modeL INFLOW HYDROGRilPHS Since only the peak discharge for each recurrence interval was given, inflow hydro graphs were created in order to incorporate the timing of the overcopping into the Cooper Slough SWNfM modeL The five overflow locations are all in the same hydrologic: sub-basin in the Boxelder SWMM model (sub-basin 53). Therefore, the inflow hydrographs a.t the overtopping 10catioIl5 were determined by multiplying the Boxelder runoff hydrograph at State Highway 14 (which is the design outfall point for sub-basin 53) by the ratio of flow over the highway to the total Boxelder Creek flow at Sta::e Highway 14. At two 10catioIl5 the overflows were combined, At tile Vme Driv~~ intersection with 1-25 the Larimer-Weld Ditch overflclw was combined with the Vine Drive overflow. This is the upstream study limit. Also, the overflow at the low point of the highway south of Vme Drive was combined with the C&S railroad underpass flow on the west side of 1-25 at the railroad, Therefore, inflow hydro graphs were created at three locations, V mE: Drive, the C&S railroad underpass, and State Highway 14, In appendix B the inflow hydrographs are plotted for each recurrence interval and the discharge values are listed at 5-minute int(:Iilals. Inflow hydro graphs for the SWMM model can only have 20 discharge values. Therefore, the 20 most n:presentative discharge values out of the :.80 listed for each hydro graph were slelecte:d.